The present invention relates to a method for controlling the air-to-fuel ratio in an internal combustion engine in which the air-to-fuel ratio is determined from components in the exhaust gas expelled from the engine.
In the prior art, to control the air-to-fuel ratio, a method such as the following has generally been employed. An oxygen sensor is used to detect the air-to-fuel ratio from exhaust gas components of the engine. The output of the oxygen sensor is compared with a predetermined voltage. According to the result of this comparison, the integration direction of an integrator is controlled. The rate at which fuel is supplied to the internal combustion engine is then varied in proportion to the output of the integrator to control the air-to-fuel ratio.
The method described above has found wide use. However, the method is disadvantageous in that if the oxygen sensor fails or the electrical connections thereto are broken, the output signal from the oxygen sensor will no longer correspond to the desired variations of the air-to-fuel ratio. As a result, the integration function is performed only in one direction, whereupon the air-to-fuel ratio becomes extremely large or small (lean or rich) to the point that the engine may stall.
This difficulty may be overcome by limiting the width of variation (the feedback control width) of the integrator. In this case, different air-to-fuel ratios are set for different engines by an open loop technique in accordance with various parameters of the engine. However, using this technique, if the air-to-fuel ratio is on the lean side, it is considerably difficult to perform feedback control to shift the air-to-fuel ratio towards the rich side. That is, the controllability of the air-to-fuel ratio is less than desirable.